Electrochemical sensor with solid phase electrolyte

ABSTRACT

An electrochemical sensor comprising an electrolyte and an analyte separated by a selectively permeable membrane preferably characterized in that the electrolyte is an electrically conductive solid comprising a homogeneous dispersion of a polymeric matrix phase and an electrically conductive salt and substantially free of water. Preferably the polymeric matrix phase is plasticized, the plasticizer forming a continuous phase in which the conductive salt is dissolved. The sensor is used, for example, for sensing and measuring gases especially in transcutaneous measurement of blood gases.

This is a continuation-in-part of application Ser. No. 07/049,902, filedMay 15, 1987, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to electrochemical sensing devices utilizingmembranes to separate the electrolyte needed by the device from themedium containing the analyte of interest. More specifically, it relatesto: (1) gas sensing devices wherein the membrane is utilized to separatethe electrolyte from a liquid containing the gas being analyzed, (2) gassensing devices wherein the membrane is utilized to separate theelectrolyte from a gas phase, and (3) ion sensing devices wherein aselectively permeable membrane separates the liquid electrolyte utilizedby the device from the analysis solution, which contains an analyte ableto diffuse freely through the membrane.

Included in gas sensing devices wherein the membrane is utilized toseparate the electrolyte from a liquid containing the gas being analyzedare the sensors utilized for transcutaneous blood gas monitoring, forclinical laboratory analysis of blood gases, and for laboratory andfield measurement of gases such as ammonia, carbon dioxide, oxygen andchlorine. Also included in this category are intravascular (catheter)electrodes with proximal or remote reference electrodes for measuringoxygen or carbon dioxide.

Gas sensing devices wherein the membrane is utilized to separate theelectrolyte from a gas phase include electrochemical sensor utilized formeasuring gases such as oxygen, carbon dioxide, chlorine, or ammonia inflowing gas streams. Such devices are currently utilized for verifyingthe oxygen content of gas mixtures used for respiratory therapy.

Ion sensing devices wherein a selectively permeable membrane separatesthe liquid electrolyte utilized by the device from the analysis solutionwhich contains an analyte able to diffuse freely through the membrane,include, specifically, intravascular electrodes for measuring blood pH,sodium, potassium and glucose.

The invention consists of a method and apparatus to simplify the processof changing the membrane and the electrolyte needed by theelectrochemical sensing device. While it is believed that the majorvalue will be with gas sensing devices, the invention can be used withany of the electrochemical devices described above which use replaceablemembranes.

Basic to the operation of electrochemical sensors is the presence of anelectrolyte, an ionically conducting medium, contacting both the anodeand the cathode. In voltammetric oxygen sensors of the Clark type orpotentiometric carbon dioxide sensors of the Stow-Severinghaus type orother gas sensors of the types disclosed by Ross and Riseman, thiselectrolyte is an aqueous solution, sometimes modified by otherwater-compatible solvents such as ethylene glycol or propylene glycol orglycerol. At times, these other solvents may make up the bulk of theelectrolyte solution and the water content may vary from traces to onlya few percent.

The membrane in these devices may serve several functions. It can beutilized to prevent evaporation of the electrolyte solvent or to preventfouling of the electrodes. It can prevent contamination of theelectrolyte solution or changes in the solution concentration. It can beselectively permeable, allowing only gases to enter for analysis, or itmay allow ions and not proteins to reach the sensing electrodes. It canbe a diffusion barrier and provide most of the concentration gradientbetween the medium being analyzed and the electrode where the analyte isbeing consumed. It can control the thickness of the electrolyte layerand, under some conditions, control the sensitivity of microelectrodes.

The output stability, i.e., the ability to maintain a reproducibleoutput signal for periods ranging from hours to days when the sensor isexposed to a reproducible gas composition, in both voltammetric andpotentiometric sensors, is dependent upon the maintenance of a constantcomposition in this electrolyte, although the reason for this isdifferent in the two types of sensors.

In voltammetric sensors, e.g., oxygen, stability is dependent uponmaintaining the constancy of the diffusional pathways to the cathode.This means that both the geometry of the diffusion layer and any of itsproperties which affect transport, such as solubility and diffusioncoefficient, should remain constant; the response of an oxygen sensor isrelatively insensitive to the absolute concentration of any of the ions.With potentiometric sensors, on the other hand, the concentration of oneor more of the ions is critically important; for carbon dioxide sensors,a common stability specification requires that concentration changes belimited to less than one percent per hour.

While many factors may have a significant effect upon electrolyteproperties, an important one is the diffusion of water vapor through themembrane. This directly affects the concentration of electrolytes, soaffecting the readings of potentiometric gas sensing electrodes, and forelectrolytes with a low water content, changes may have a strong effectupon the diffusional properties of the electrolyte and so affectvoltammetric sensors. Although membranes are typically made from ratherhydrophobic polymers, good transient response and a number ofengineering considerations have limited the number of satisfactorymaterials to a small number with intermediate transport properties foroxygen and carbon dioxide and concomitantly, for water vapor.

The need for good transient behavior limits the thickness of theelectrolyte layer between the membrane and the sensing electrode. Formany electrode configurations, such as transcutaneous devices andintravascular devices, it has heretofore proven impossible to provide alarge electrolyte reservoir, and, therefore, the electrolyte has had tobe replenished at regular intervals.

Historically, membrane-based electrochemical sensing devices have beenprepared for use by placing a small volume of electrolyte so that it iscontained between the surface of the structure which contains theelectrodes and the membrane which separates the sensor and itselectrolyte from the medium being measured. The membrane may be fixed inposition by a variety of means, ranging from a rubber band or O-ring toa structure which holds the membrane and which can be fastened to thesensor by screw threads, an interference fit, or an over center,snap-like device. Mechanical aids may be used to facilitate assembly.

With electrolytes used heretofore, the user should add a small volume,usually less than a milliliter and often a drop or less, of theelectrolyte to the electrode face or the membrane surface, and thisusually requires a fresh membrane because the previous applicationstretched the membrane enough so that reapplication produces a loosefit. There are several other reasons why membranes have to be replacedat regular intervals--mechanical damage, membrane fouling, the need topolish the electrode surface (in the case of oxygen sensors), andelectrolyte evaporation. While changing a membrane is not a difficultoperation, it is not only an inconvenience but a continuing potentialsource of operational errors, and in the clinical environment,especially, any simplification has value.

SUMMARY OF THE INVENTION

In order to overcome at least some of the problems of prior art(electrochemical sensors which use fluid phase electrolytes), thepresent invention teaches the use and construction of an electrochemicalsensor having an electrode, an electrolytic solution in a substantiallysolid phase at ambient temperatures in electrical contact with saidelectrode, and a selectively permeable membrane in contact with saidelectrolytic solution for selectively passing an analyte into saidelectrolytic solution. More specifically, the invention includes anelectrochemical sensor wherein the electrolyte is an electricallyconductive solid comprising a homogeneous dispersion of a polymericmatrix phase and an electrically conductive salt. Very desirably, thepolymeric matrix phase is plasticized, the platicizer forming acontinuous phase in which the conductive salt is dissolved. Thepreferred polymer is a polyvinyl alcohol and the preferred plasticizertherefor is a polyhydric alcohol or mixture thereof. What is novel andinventive herein lies at least partly in the use in an electrochemicalsensing device as defined above of a solid phase electrolyte which issubstantially free of water, thereby avoiding variability in the sensorthrough gradual evaporation of water during use of the sensor. Moreover,the selectively permeable membrane can be replaced easily. In at leastthe best embodiments of the invention, a good transient response isobtainable, and therefore, the problem of electrolyte replenishment isalso overcome.

The invention further includes a method of making an electrochemicalsensor of the invention, comprising:

(1) combining, in any order, ingredients including a polymer and anelectrically conductive salt (and preferably also a plasticizer and avolatile solvent for said polymer and plasticizer, to form a homogeneousmixture containing the polymer as a matrix phase and when a plasticizeris included a continuous phase thereof in which the conductive salt isdissolved,

(2) forming said mixture into a thin film,

(3) affixing said film to a membrane,

(4) mounting said membrane adjacent said electrode and

(5) when said ingredients include a volatile solvent, before or afterstep (3) drying said film to evaporate substantially all said volatilesolvent.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation view illustrating an electrochemicalsensor according to a preferred embodiment of the invention.

FIG. 2 is a graphic view illustrating the response of an electrochemicalsensor according to one preferred embodiment of the invention.

FIG. 3 is a comparative graphic view illustrating the response of anelectrochemical sensor according to another preferred embodiment of theinvention.

FIG. 4 is another comparative graphic view illustrating the response ofan electrochemical sensor according to another preferred embodiment ofthe invention.

FIG. 5 is a comparative graphic view further illustrating the responseof an electrochemical sensor in its intended environment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is the purpose of this invention to describe a series of solidpolymer based electrolyte systems which will eliminate many of theproblems associated with the limited life of the electrolyte and providea system for renewing the electrolyte without the need to handle aseparate liquid. This can be done in two ways: (1) by utilizing a solidpolymer based electrolyte which is adherent to the membrane; and (2) bypreparing a solid polymer based electrolyte which can be handled withouta supporting membrane. It can be applied to an electrode in a fashionpreviously known, utilizing a membrane-handling structure such as aNovaDisk membrane attachment device manufactured by Novametrix MedicalSystems, Inc., Wallingford, CT 06492 and more fully described in U.S.Pat. No. 4,280,505 to Dali et al.

There are at least four types of polymer-based systems which can be usedto formulate solid polymer based electrolytes for electrochemicalsensing devices. These may be described as follows:

(1) Polymer-gelled electrolyte system, wherein well known standardelectrolytes bases upon water, glycols, etc. can be converted intosemi-rigid solids by using certain water-soluble polymeric gellingagents.

(2) Polymer-plasticiser system based upon plasticisers with highdielectric constants which can be utilized with compatible polymers toproduce solid polymer electrolytes usable with intravascular andtranscutaneous sensors. These do not have the high resistancecharacteristc of previously known systems, such as those based upon PVCand other polymers with relatively low polarity; therefore, they makeeffective solid electrolytes for use with oxygen sensors.

(3) Polymer-salt complex system based on polyethylene oxide andderivatives, with sufficiently high conductivity to be effective solidpolymer electrolytes for oxygen sensors. Their successful utilization insensors is dependent upon modifications which will provide high rates ofdiffusion and the incorporation of anions which will support anodereactions and stabilise reference electrodes for voltametric sensors andpotentiometric sensors, respectively.

(4) Ion exchange membrane system. A solvent-soluble, cation exchangepolymer (e.g., sold by DuPont under the trademark Nafion) can be usedwith a non-volatile solvent to provide a solid polymer-based electrolytesystem suitable for use with electrochemical sensors. Normallyconductivity is very dependent on water content and oftenapproaches-zero in the air-dry state, but with a non-volatile solvent,adequate conductivity does not require water.

An ionically conducting solution (an electrolyte) is a salt solution; inorder for salts to readily dissolve and ionize in a solvent, the solventshould have a high dielectric constant. With only a small number ofexceptions, solvents with high dielectric constants are also extremelywater soluble. They are generally poor solvents for polymers and notcompatible with them. Identified herein are specific polymer-solventsystems with the desired combination of properties and the generalclasses of materials with the desired properties.

A solid polymer electrolyte system should give a plasticiser to act as asolvent for salts, a salt to provide for ionic conductivity, a polymerto make the system solid, and any necessary fabrication aids.

The basic function of the plasticizer is to act as an ionizing solventfor the salts to provide ionic conductivity and to provide the ionsneeded by the anode reaction or to stabilise the potential of thereference electrode. Although salts are generally insoluble innon-aqueous solvents, the plasticizer should be selected from thosesolvents that can be expected to have solubility for a reasonablevariety of salts. This implies a high dielectric constant for theplasticizer. In addition, the plasticizer should be substantiallynon-volatile and not electrochemically reducible, and it should notenhance the solubility of silver chloride.

The plasticizer should swell the polymer and have sufficientcompatibility so that no phase separation occurs. Although plasticizerlevels as low as 30% may be acceptable, a variety of informationsuggests that plasticizer levels as high as 80-85% may be desirable. Theplasticizer should not have any deleterious effects upon any of thematerials used in the construction of the electrode.

The plasticizer will normally have a boiling point at 25° C. and 1atmosphere pressure of at least 225° C., preferably at least 250° C. (oran equivalent lower boiling point at reduced pressure). When PVA is thepolymer, the preferred plasticizer comprises an aliphatic orcycloaliphatic polyhydric alcohol having 2 to 8 carbon atoms, or mixturethereof having an average from 2 to 4 hydroxyl groups per molecule,preferably more than 2 and less than 4. Other known plasticizers forPVA, typically having hydroxy, amine or amide groups can be used.

The basic functions of the salts will vary with the nature of thesensor. For potentiometric sensors, one of the salt ions should bereversible at the reference electrode to stabilize its potential.Usually, the reference electrode is a silver/silver halide electrode inwhich case the anion is desirably halide ("halide"=chloride or bromide).Other types of reference electrodes are conceptually possible.

For voltammetric sensors, salts should support the anode reaction andprovide adequate conductivity to prevent excessive IR drop in theelectrolyte as is necessary to prevent nonlinearity. Adequateconductivity implies reasonable solubility and ionic mobility.Solubility should not be obtained by sacrificing ionic conductivity. Theplasticizer should be selected to have a dielectric constant high enoughto prevent excessive ion-pair formations at a selected suitable saltconcentration. The halide concentration should be held to a minimum, inorder to limit silver transport in voltammetric sensors. The bulk of theconductivity can be provided by an indifferent salt which does notparticipate in any of the electrode reactions. The salt should not reactat the cathode or accelerate any degradation reactions. The salt shouldnot enhance the solubility of the silver halide used as the referenceelectrode.

The basic function of the polymer is to provide a solid matrix for theplasticizer electrolyte system. The polymer should be compatible withthe plasticizer at levels high enough so that a continuous plasticizerphase exists within the film. The polymer should impart enough strengthso that the film will withstand the handling needed to manufacture andstore the device. The polymer should have adequate long term stability.and it should not interfere with any of the electrode reactions. Itshould have adequate purity and not increase the solubility of thesilver halide. The polymer-plasticiser-salt system should have adequateconductivity.

In general, when the polymer is plasticized it should be one which isreadily solvatable by a plasticizer and have a pronounced tendency tohydrogen bond formation. These preferred characteristics can beexpressed with reference to a solubility parameter and to either or bothof two hydrogen bonding parameters known as Crowley's parameter γ_(c)and Hansen's parameter δ_(H). These three parameters are to be found inthe CRC Handbook of Solubility Parameters, CRC Press 1983. Table 5 onpages 153-158 and Tables 14 and 15 on pages 186-190. The solubilityparameter of the polymer is preferably 24 or more, most preferably 27 ormore (the value of triethylene glycol is 27.5) and still better at least30, the units being megaPascals^(1/2). (The value for PVC is only 19).As to H-bonding parameters, pH is preferably greater than 17(triethylene glycol=18) and γ_(c) is preferably greater than 18.Although values for the most preferred polymer, polyvinyl alcohol(hydrolyzed polyvinyl acetate), are not given in this book, triethyleneglycol provides a useful marker, indicating that the values for PVAwould be higher. Preferably the plasticizer and polymer have about thesame solubility and hydrogen-bonding parameters. Other polymersconforming to these preferred requirements are polyvinylidene chlorideand, polyacrylonitrile and various copolymers of the monomers of any ofthese polymers with minor amounts of appropriate co-monomers, especiallyvinyl chloride-hydrolyzed vinyl acetate copolymers.

A surfactant may be necessary, because it is highly desirable tocontinue to use Teflon as a membrane material. In general, it isdesirable to use a non-ionic surfactant to avoid compatibility problemswith salts. The surfactant should be compatible with both theplasticizer and the polymer. Depending on the film formation processused, other additives, such as stabilizers and internal lubricants maybe necessary.

A solid polymer electrolyte can be prepared by combining, in any order,a polymer, a non-volatile solvent (plasticizer), a salt, and processingaids, such as surfactant, antioxidant, or lubricant. This is renderedinto a homogeneous mixture by heat and/or with the aid of a volatilesolvent. This homogeneous mixture is converted to a thin, solid,electrolyte film and adhered to a gas-permeable, water-impermeablemembrane by various methods, detailed below.

A thin solid polymer electrolyte film may be made by extrusion or bycasting followed by evaporation of the volatile solvent. This thin solidpolymer electrolyte film may be affixed to the membrane by laminating orroll-bonding, for combining two or more polymeric materials into asingle thin film. Discs of this combination material may then beincorporated into fixation rings.

The thickness of the solid electrolyte depends on the intended use. Fora transcutaneous sensor, it is desirably less than 25 micrometers. Forother sensors, it can be somewhat greater, although the transientresponse (the time taken for the device to equilibrate to 90% of itstotal change) is approximately proportional to the square of thethickness, so 50 or perhaps 75 micrometers would be a normal upper limitof thickness.

Alternatively, a disc of this solid polymer electrolyte film may be cutand then affixed to a selectively permeable membrane which is thenincorporated into a fixation ring, or the disk may be applied to themembrane which has previously been assembled into a fixation ring.

Alternatively, the homogeneous mixture can be applied directly to thepermeable membrane material by casting, transfer printing, extrusion,calendering, or other surface coating operation followed by solventevaporation or cooling to solidify the polymer electrolyte. Thispermeable membrane-electrolyte combination can then be incorporated intothe fixation ring assembly. In the preferred embodiment, the homogeneousmixture is applied by casting or printing to limited areas of theselectively permeable membrane, which have been previously assembledinto fixation rings.

A fixation ring suitable for use with a solid electrolyte as describedherein is disclosed in U.S. Pat. No. 4,280,505 to Dali et al. which isassigned to the assignee of this patent. The structure of anelectrochemical transcutaneous sensor which includes a solid phaseelectrolyte is described briefly below with reference to FIG. 1. Thestructural elements of the sensor, other than the electrolyte are morefully described in U.S. Pat. No. 4,280,505 which is, herein,incorporated by reference.

Referring now to FIG. 1, there is shown a transcutaneous gas sensorprobe 1 including a housing 3 in which there is mounted an anode 5 and acathode 7. The housing 3 has an enlarged cylindrical upper portion and asmaller diameter lower portion with threads 17 on its exterior. Afixation ring 18 has a housing 19 with interior threads for mounting thefixation ring 18 on the housing 3. Membrane 29 is mounted on thefixation ring 18 as described therein. The membrane 29 may be formedfrom a material which is permeable to a gas intended to be measured,e.g., polypropylene for oxygen and teflon for carbon dioxide, with thesolid electrolyte affixed to its upper surface as herein set forth. Themembrane 29 may also be formed from a thin disc shaped layer of thesolid electrolyte. Alternatively, the membrane 29 may comprise two ormore layers, one of which is selectively permeable to the gas understudy and another of which comprises a sheet of the electrolyte.

EXAMPLES

FIG. 2 is a graphic view illustrating the response of a conventionaltranscutaneous oxygen electrode covered with a membrane utilizing asolid polymer electrolyte, based upon poly(vinylidene) chloride. Itshows the initial rapid downward drift in air and the substantialstabilization in less than 10 minutes. It was then exposed to low gas(5% carbon dioxide in nitrogen) gradually stabilized to a reading nearzero, and then showed an appropriate response to high gas (10% carbondioxide, 12% oxygen, balance nitrogen).

Curve B in FIG. 3 is a graphic view illustrating the response of atranscutaneous oxygen electrode having a corona-treated Teflon filmcoated with a solid polymer electrolyte, based upon poly(vinylalcohol)according to one example. Curve A illustrates the response of a similarelectrode utilizing a conventional liquid electrolyte.

Examples of suitable polymers for a solid electrolyte system includepolyethylene oxide-magnesium chloride complex, polyethyleneoxide-lithium salt complexes, poly(ethylene succinate)-lithiumperchlorate, -lithium thiocyanate, or -lithium fluoroborate complex, andpoly(2-methoxyethyl, polyethylene glycol)methacrylate-alkali metal saltcomplexes, poly(perfluoroethylene sulfonic acid) andpoly(perfluoroethylene carboxylic acid).

Plasticizers may include propylene carbonate, ethylene carbonate,polyethylene glycol, glycerol,2-ethyl-2-(hydroxymethyl)-1,3-propanediol, 2-ethyl-1,3-hexanediol,1,2,6-hexanetriol, 2,2-diethyl-1,3-propanediol,1,1,1-tris(hydroxymethyl)ethane, gamma-butyrolactone,1,3-cyclohexanediol, and 1,4-cyclohex-anedimethanol.

Salts may include lithium nitrate, lithium chloride, magnesium nitrate,magnesium chloride, tetrabutylammonium nitrate, tetrabutyl ammoniumbromide, tetrabutylammonium chloride, tetraetheylammonium nitrate,tetraethylammonium perchlorate, tetraethylammonium chloride,tetraethylammonium bromide, potassium nitrate, and potassium acetate.

Surfactants may include DuPont Zonyl surfactants FSP, and FSN, and 3MFluorad Surfactants FC-170-C and FC-93.

The manufacturer's data define these surfactants as follows. DuPont FSNis F(CF₂ CF₂)₃₋₈ CH₂ CH₂ O)H. DuPont FSP is (F(CF₂ CF₂)₃₋₈ (CH₂ CH₂O)₁,2 P(O)(ONH4)₂,1. 3M FC-93 is ammonium perfluoralkyl sulfonates and3M FC-170-C is fluorinated alkyl polyoxyethylene ethanols.

A. GELLED ELECTROLYTE SYSTEM

To 100 ml of a 95% ethylene glycol-water mixture at or below roomtemperature, add gradually, with good stirring, 0.75 g of Viscarin, aKappa-II carrageenan provided by the Marine Colloids Div., FMCCorporation, Springfield, NJ 07081. When this is thoroughly dispersed,warm it on a hot water bath, with stirring, until it is a uniformsolution. Continue heating until it is 75 degrees C., transfer to ablender, add 2.55 g NaNO₃ and 1.0 ml 1.0M NaCl solution and blend untilit appears homogeneous. Reheat to 75 degrees C., add 1.0 ml 2.56M KHCO₃solution and blend again. To half this solution, add 0.15 ml Zonyl FSP35% solution (an anionic fluorosurfactant) provided by the DuPont Corp.,Wilmington, Delaware. Blend until homogeneous. Maintain at 75-80 degreesC. without stirring until the solution is homogeneous. While thesolution is at the above specified temperature, centrifuge in a clinicalcentrifuge until it is clear and free of all bubbles. Upon cooling toroom temperature, this material sets to a soft gel.

This carrageenan-based gel melts at 40-60 degrees C., so that very smalldrops can be dispensed onto the interior surface of a membrane mountedin a NovaDisk membrane attachment device used for a transcutaneouselectrode. When the fluid has cooled to room temperature, it gels againand can be handled and shipped without any flow. When applied to thesurface of an electrode at operating temperature, it flows well enoughto form a thin film, providing normal operation.

Samples of these membranes were clinically tested with satisfactoryresults.

There is no need to limit the solvents to those that have historicallybeen used in gas sensing electrodes. Solvents less volatile thanethylene glycol and water can be used if their diffusional propertiesare acceptable. This greatly expands the number of potential gellingagents because they do not have to be water soluble. Although notgelled, a system utilizing a mixture of propylene carbonate andpolyethylene glycol 1000 showed excellent transient response andsensitivity.

Other polymers that are suitable for gelling the wide range ofplasticizers that are potential candidates as non-volatile electrolytesolvents include materials such as polyacrylamide and modifiedcelluloses, such as sodium carboxymethyl-cellulose,hydroxypropylcellulose, etc. There are well known methods to crosslinkand, therefore, gell, all of these polymers.

B. POLYMER-PLASTICISER SYSTEM Example 1

Saran F278, a proprietary co-polymer, predominantly poly(vinylidenechloride) (Dow Chemical Co., Midland, MI), 2.038 gm and propylenecarbonate 2.071 gm were added to 3.012 gm tetrahydrofuran (THF) and1.494 gm xylene and dissolved at 65° C. To half this solution, 16 mg oftetraethylammonium chloride was added; this yielded a clear solution.This was cast on to a glass plate and dried at 65° C. The resistancemeasured with an AC conductivity bridge, was 500,000 to 1,000,000 ohms.

A 0.25 inch (6.3 mm.) diameter circle was cut from this material. It wasmounted on the face of a transcutaneous oxygen sensor covered with astandard NovaDisk membrane attachment device. The sensor responded tooxygen.

Example 2

Saran F310, a proprietary co-polymer, predominantly poly(vinylidenechlorides (Dow Chemical Co., Midland, MI) 1.584 gm and 2.418 gmpropylene carbonate solution which is 0.3M in tetraethylammoniumperchlorate and 0.01M in tetraethylammonium chloride were added to 6.590gm methylethylketone. After warming at 65° C. for two hours withultrasonic agitation, this yielded a homogeneous, more or less clearsolution. This was cast onto glass and dried at 65° C. for one hour.This produced a clear, tough, highly stretchable film.

A sample of this solution was cast onto a corona-treated Teflon film andmounted on a transcutaneous oxygen electrode.

Example 3

Poly(vinylidene fluoride) CAT #102 (Scientific Polymer Products, Inc.,6265 Dean Parkway, Ontario, NY 14519) 1.789 gm and 2.435 gm of apropylene carbonate solution which is 0.3M in tetraethylammoniumperchlorate and 0.01M in tetraethylammonium chloride, were added to6.727 gm of cyclohexanone. After warming at 65° C. for two hours withultrasonic agitation, the polymer had not completely dissolved, so themixture was placed in a boiling water bath at 100° C. It slowlydissolved, yielding a highly viscous solution, which gelled on coolingto room temperature overnight. The solution was diluted by adding 2.018gm of cyclohexanone and heated on the boiling water bath until it washomogeneous. The polymer solution at 100° C. was cast on to a glassplate at 65° C. After drying at 65° C. for 1.5 hours, a cloudy film wasobtained which gave a high response upon loose application to the faceof a transcutaneous oxygen electrode.

Example 4

Poly(acrylonitrile) (PAN) Cat #134, nominal M.W. 150,000, was obtainedfrom Scientific Polymer Products, Inc., 6265 Dean Parkway, Ontario, NY.A solution was prepared by taking 5.0 ml of a 9.5% solution of PAN indimethylformamide (DMF), and adding 2.415 gm of a propylene carbonatesolution 0.3M in tetraethylammonium perchlorate and 0.01M Intetraethylammonium chloride. The mixture was heated at 75°-85° C. untilit was a homogeneous solution. This material, cast on glass and dried at65° C., yielded a plasticized film. When placed on a transcutaneousoxygen electrode, a response of 1.77 pA/Torr was obtained.

Example 5

Poly(vinyl alcohol) (PVA) Cat #334, MW 125,000, 88% hydrolysed wasobtained from Scientific Polymer Products, Inc., 6265 Dean Parkway,Ontario, NY. A 7.44% solution of polymer in water was used to prepare avariety of films. For example, 2.69 ml of this solution was taken. Toit, the following plasticizer solutions were added: 1.2 ml of a 25%solution of trimethylol propane in water and 1.2 ml of a 25% solution ofglycerol in water. For salts, there were added 0.6 ml of 1.0M tetrabutylammonium nitrate (TBNO₃) end 0.6 ml of 0.1M tetrabutyl ammonium chloride(TBACl). To promote better casting, 0.1 ml of a 2% solution of DuPontFSN surfactant was added. This solution was heated to ensure uniformityand then cast on the inner surface of a NovaDisk membrane attachmentdevice containing a Teflon membrane. After drying at 65° C. for 1.0hours, this was applied to a transcutaneous oxygen electrode. FIG. 4shows the initial 15 hours of the response of this electrode withvarious gases on a calibration device during a laboratory test, to air,low gas, high gas, and back into air. Curve A shows a conventionalelectrode utilizing a liquid electrolyte, and Curve B shows a similarelectrode with a solid polymer electrolyte, based uponpoly(vinylalcohol). FIG. 5 shows the response of another electrode witha different solid polymer electrolyte upon application of the sensor tothe skin of a volunteer. Curve A is a control utilizing a conventionalliquid electrolyte, and Curve B is an electrode with a solid polymerelectrolyte.

A variety of other materials have demonstrated utility as plasticisers.These include propylene carbonate,2-ethyl-2-(hydroxymethyl)-1,3-propanediol, 2-ethyl-1,3-hexanediol, andseveral others.

From the foregoing examples, it can be concluded that many aliphaticsubstances with molecular weights less than two hundred, and/or having 2to 8 carbon atoms, bearing two or more hydroxyl groups, but normally notmore than four will show suitably low volatility and compatibility withthese and other suitable polymers. It is not even necessary for thesematerials to be liquid at room temperature. One of the examples givenabove, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol, is a solid at roomtemperature. These substances may also include in their structure estergroups (as in propylene carbonate) and/or ether groups (as intriethylene glycol). Compounds containing these structures have the highdielectric constants and polarity necessary to solvate and, therefore,dissolve and ionize salts.

In addition to the poly(vinyl alcohol) described above, various othergrades of that polymer have shown utility. These include materials withmolecular weights down to 25,000 or even to 5,000 and with polyvinylacetate hydrolysis levels ranging from 75% to 99%. These limits are notspecified as being inclusive but merely represent the limits ofmaterials that were readily available for testing.

Various other film-forming polymers have the chemical properties neededto produce solid polymer electrolytes. These include various cellulosederivatives, such as sodium carboxymethylcellulose andhydroxypropylcellulose, polyacrylamide, poly(hydroxyethyl methacrylate),etc. Both of the cellulose derivatives are known to be plasticized byone or more of the plasticizers listed above.

A variety of salts have demonstrated satisfactory results. These includemagnesium nitrate and chloride, potassium acetate, sodium bicarbonate,chloride, and nitrate, tetrabutylammonium chloride and nitrate, andtetraathylammonium chloride, nitrate and perchlorate. From this, it canbe concluded that any salt soluble in the plasticizer may givesatisfactory results.

It is also possible to cross-link the polymer by various chemicalreactions; this is one advantage that PVA has over other polymers. Itcan be cross-linked by several different chemical reactions. Thesimplest of these is the addition of borate ion, and this has been done.

Example 6

A solution was made of 1.0 ml 9.6% PVA solution (88% hydrolyzed, 25,000molecular weight). 1.2 ml of a plasticizer solution containing 12.5%glycerol and 12.5% 2,2-diethyl-1,3-propane-diol. 1.56 ml of a saltsolution that was 0.238M in Mg(NO₃)₂, 0.0038M in MgCl₂, and 0.0096M inlithium borate. This solution was cast on glass, dried at 65 degrees C.,and yielded a tough, highly elastic film. A small volume, about 10microliters, was cast on the interior of a NovaDisk membrane attachmentdevice membrane, dried at 65° C., and applied to a transcutaneous oxygenelectrode. It gave a response of 2.5 to 3.2 pA/Torr in air.

It has recently been shown that the helical structure of polyethyleneoxide will complex alkali metal ions, producing salt-polymer complexeswith relatively high conductivity. These materials, particularly ifplasticized with compatible low molecular weight materials, showsufficient electrical conductivity and gas permeability to function asan electrolyte in electrochemical sensors. Specifically, a highmolecular weight polyethylene oxide could be plasticized with any of theplasticizers disclosed above. Since most of these plasticizers willdissolve and ionize most of the salts listed above, adequateconductivity is obtainable.

A recent publication, "Solid Polymer Electrolyte Complexes of AlkaliSalts with Poly(methacrylate)s Carrying Pendant Glyme Chains," PolymerPreprints, 25, no. 2, 107 (1984), describes a polymethacrylate withcovalently bonded polyethylene oxide side chains which shows significantconductivity. The authors also note that the addition of propylenecarbonate to the polymer leads to greatly increased conductivity. Asnoted above, the vapor pressure of propylene carbonate is too high forpreferred use with many polymers in an electrochemical sensor, and otherplasticizers of the sort described above can be expected to be equallyeffective and more resistant to evaporation.

Nafion is a fluorinated, sulfonic-acid, ion exchange polymer which issoluble, by proper treatment, in mixtures of water and the loweralcohols. When a film is cast from this solution and dried, it hasexcellent resistance to redissolving in water. It has excellentconductivity and gas permeability but only when it contains considerablewater. This is also expected when it is solvated with plasticizer.Dissolution of the salts increases conductivity when this polymer isutilized in highly concentrated solutions where the total electrolyteconcentration in the membrane phase can be expected to be very high.

Salts should be soluble in one or more of the above plasticizers. Theyshould include an ion which can stabilize the reference electrode inpotentiometric sensors or provide for an anode reaction in voltametricsensors. The salts should not be reducible at the cathode nor increasethe solubility of the anode in voltametric sensors. The salts shouldprovide sufficient conductivity so that the voltage drop through theelectrolyte in voltametric sensors does not cause excessivenonlinearity. Salts which have been found satisfactory include lithiumnitrate and chloride, magnesium nitrate and chloride, tetrabutylammoniumnitrate and chloride, tetraethylammonium nitrate, perchlorate, andchloride, potassium nitrates and potassium acetate. It is reasonable tosuppose that a great many other alkali and alkaline earth ions will havesalts showing the requisite properties.

To prepare a solid polymer electrolyte by casting an aqueous solutiononto a Teflon film, it is necessary to utilize an extremely effectivesurfactant. Materials that have been used include DuPont Zonylsurfactants FSP and FSN; these are both fluorosurfactants. Otherpreparation methods may require other processing aids.

Solid polymer electrolyte may be prepared by casting the mixture from asuitable solvent or directly from a mixture of the components byextrusion, by injection moulding, and by plastisol technology, i.e., byheating a suspension of finely divided polymer in a plasticizer solutionof the salts and any needed processing aids, such as surfactants,antioxidants, etc.

It is to be appreciated that variations and modifications may be made tothe preferred embodiments of the invention disclosed herein withoutdeparting from the spirit and scope of the invention.

I claim:
 1. An electrochemical sensor comprising an electrolyte and an analyte separated by a selectively permeable membrane where the electrolyte comprises an electrically conductive solid comprising a homogeneous dispersion of a polymeric matrix phase, where the polymeric matrix phase is plasticized, and an electrically conductive salt said dispersion being substantially free of water, the plasticizer forming a continuous phase in which the conductive salt is dissolved.
 2. A sensor according to claim 1 where the polymetric phase is a polyvinyl alcohol.
 3. A sensor according to claim 2 where the polymetric phase is a hydrolyzed polyvinyl acetate having a degree of hydrolysis of from 80 to 99% and a molecular weight of at least 5,000.
 4. A sensor according to claim 3 where the plasticizer comprises an aliphatic or cycloaliphatic polyhydric alcohol having from 2 to 9 carbon atoms or a mixture of the least two alcohols having an average of from 2 to 3 hydroxy groups.
 5. A sensor according to claim 3 where the plasticizer comprises a mixture of glycerol with 2-ethyl-2-(hydroxymethyl)-1,3-propanediol or 2,2-diethyl-1,3-propanediol.
 6. A sensor according to claim 2 where the plasticizer comprises an aliphatic or cycloaliphatic polyhydric alcohol having from 2 to 8 carbon atoms or a mixture of at least two alcohols having an average of from 2 to 3 hydroxy groups.
 7. A sensor according to claim 2 where the plasticizer comprises a mixture of glycerol with 2-ethyl-2-(hydroxymethyl)-1,3-propanediol or 2,2-diethyl-1,3-propanediol.
 8. A sensor according to claim 1 where the polymetric phase has a solubility parameter of at least 24 megaPascalsq, and a hydrogen bonding parameter γ_(c) (Crowley) greater than 18 or δ_(H) (Hansen) greater than
 17. 9. A sensor according to claim 8 where the plasticizer has about the same solubility parameter and hydrogen bonding parameter as the polymetric phase.
 10. A sensor according to claim 9 where the plasticizer has a boiling point of at least 225° C. at 25° C. and 1 atmosphere.
 11. A sensor according to claim 9 where the boiling point is at least 250°.
 12. A sensor according to claim 8 where the plasticizer has a boiling point of at least 225° C. at 25° C. and 1 atmosphere.
 13. A sensor according to claim 8 where the boiling point is at least 250°.
 14. A sensor according to any one of claims 1 and 3-11 and 4-11 where the electrolyte has a thickness not greater than 25 micrometers. 